Harrow and Hillingdon Geological Society
Diamonds Through time
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On the human timescale diamonds were first known in southern and eastern Asia two thousand or more years ago. Their rarity made them symbols of power and they remained the property of royalty and the elite. Most came from river gravels of southern India. A few stones found their way to classical Greece and Rome but in Europe disappeared from view until the 14th or 15th century. The supply was still from Asia but by about 1700 this supply was drying up and became replaced by one from diamond-bearing river gravels in South America. The supply of South American diamonds in turn became limited in the early 19th century but in the 1860s diamonds began to be discovered in river gravels in southern Africa. These discoveries led to the ultimate source of diamonds being recognised in pipe-shaped volcanic deposits that came to be called kimberlites after the birthplace of the South African diamond industry.
Understanding of the reasons why diamonds are so rare stemmed from this discovery. Firstly their main region of formation is under areas of the continents which have remained geologically undisturbed for 2,500 million years or more (cratons). Here the diamonds formed at temperatures of ~ 1000ºC and pressures 45,000 times atmospheric, mostly at a time of three billion years ago. Secondly they needed to be transported rapidly from the depths at which they form (140-200 km) so they did not transform to graphite as pressure was released the shallower they got. It was the magmas which give rise to kimberlites and lamproites, another volcanic rock, which provided this rapid transport system at various times throughout the Earth’s history. They originate at depths of 150 km or more and are rich in carbon dioxide. They move through fractures and near the surface have explosive power as their gas content comes out of ‘solution’. However kimberlites are rare, only about 6000 pipes are known and furthermore only about one in 200 pipes contain diamonds in economic quantities, added to which only about 20% of diamonds mined are of gem quality.
Andy Fleet